New triplet silylenes M-Si-M'-X along with some unusual cyclic forms (M = Li, Na, and K; M' = Be, Mg, and Ca; X = F, Cl, and Br)

Comparison of 54 M-Si-M'-X species is carried out using quantum mechanical ab initio and DFT computations at B3LYP/6-311++G**, QCISD(T)/6-311++G**, and CCSD(T)/6-311++G** levels of theory (M = Li, Na, K; M'= Be, Mg, Ca, and X = F, Cl, Br).All triplet species with M = K appear more linear than their corresponding ones with Li and Na. The electronegativity reactivity descriptor for each halogen (X = F, Cl, Br) is used as a tool to evaluate the interrelated properties of these silylenes. Stability, assumed as singlet-triplet energy difference (Delta Es-T) for each series depends on the substituent's electropositivity, analyzed by applying appropriate isodesmic reactions. Stability of triplet M-Si-M'-X silylenes increases as functions of electropositivity of alpha-substituents and of beta-substituents. The purpose of the present work was therefore to assess the influence of different di-alkaline metals with different beta-substituents on the singlet-triplet energy gaps.